What color should spark plug tips be?

A visual inspection of the spark plug color; will display symptoms and conditions, of what may be happening, inside your engine.

So, the experienced technician, can analyze the spark plug color. Consequently, tracking down, the root cause of many problems.

Most importantly, being able to read this information properly; can be like having a window into your engine.

So, some of the symptoms, indicated by your spark plug color, have simple fixes. But, the main advantage to checking out your spark plug color is; for a quick diagnostic tool. Consequently, giving you a fairly good idea of; how well your engine is performing.

The following is a list of conditions that you may find; when checking spark plug color:

Normal Spark Plug Color

• Combustion deposits are slight and not heavy enough; to cause any detrimental effect on engine performance. Note: The brown to greyish tan color and minimal amount of electrode erosion. Furthermore, indicating the plug is in the correct heat range; and, has been operating in a “healthy” engine.

Mechanical Damage

• Usually, caused by a foreign object, that has accidentally entered the combustion chamber. It is also possible, for a small object to “travel” from one cylinder to another. May be due to, improper reach spark plugs that; permit the piston to touch or collide with the firing end.

Oil Fouled

• Too much oil is entering the combustion chamber. Caused by piston rings or cylinder walls that are badly worn. Oil may also be pulled into the chamber; because of excessive clearance in the valve stem guides. Also, if the (PCV) valve is plugged or inoperative; it can cause a build up of crankcase pressure. As a result, forcing oil and oil vapors past the rings and valve guides; into the combustion chamber.

Overheated

• So, a clean, white insulator firing tip and or excessive electrode erosion; indicates this spark plug condition. Caused by, over advanced ignition timing; poor engine cooling system, lean air fuel mixture, or a leaking intake manifold. When these conditions prevail, even a plug of the correct heat range, will overheat.

Insulator Glazing

• Glazing appears as a yellowish, varnish like spark plug color. This condition indicates that, spark plug temperatures have risen suddenly; during a hard, fast acceleration period. As a result, normal combustion deposits; do not have an opportunity to “fluff off” as they normally do. Instead, they melt to form a conductive coating, causing engine misfires to occur.

Pre-Ignition

• Usually one or a combination of several engine operating conditions; are the prime causes of pre-ignition.

Also, it may originate from:

• Glowing combustion chamber deposits.
• Hot spots in the combustion chamber.
• Poor control, of engine heat.
• Cross firing (electrical induction between spark plug wires).
• Plug heat range is, too high for the engine or its operating conditions.

Gap Bridging

• So, combustion deposits thrown loose, may lodge between the electrodes. Consequently, causing a dead short and misfire. Fluffy materials that accumulate on the side electrode, may melt to bridge the gap.

Analyzing Your Spark Plug Color, Can Tell You More Than You Think.

Most often, if your spark plugs keep fouling out; you probably have an engine problem.

Splash Fouled

• Appears as “spotted” deposits on the firing tip of the insulator. And, often occurs after a long delayed tune up. By products of combustion may loosen suddenly; when normal combustion temperatures are restored. During hard acceleration, these materials shed from the piston or valves; and are thrown against the hot insulator surface.

Detonation

• This form of abnormal combustion has, fractured the insulator core nose of the plug. The explosion that occurs in this situation; applies extreme pressures on internal engine components. Prime causes include, ignition time advanced too far. Also, lean air fuel mixtures, and insufficient octane rating of the gasoline.

Ash Fouled

• A build up of combustion deposits stemming primarily from; the burning of oil and or fuel additives during normal combustion. If heavier deposits are allowed to accumulate; they can “mask” the spark, resulting in a plug misfire condition.

Carbon Fouled

• Soft, black, sooty deposits, easily identify this plug condition. Most often caused by an over rich, air fuel mixture.
  Check for a sticking choke, clogged air cleaner, or a carburetor problem. Float level high, defective needle or seat. Also, attributed to weak ignition voltage; an inoperative preheating system, or extremely low cylinder compression.

Worn

• The voltage required to fire the plug, has approximately doubled. And, will continue to increase with additional miles of travel. Even higher voltage requirements, as much as 100% above normal; may occur when the engine is quickly accelerated. Finally, poor engine performance and a loss in fuel economy, are traits of a worn spark plug.

Spark Plug Color, Can Show Abnormal Combustion:

Pre-ignition:

• Defined as ignition of the air fuel mixture, before the pre set ignition timing mark.
• Too hot a spark plug, low octane fuel or lean air fuel mixture.
• Also, too high compression, or insufficient engine cooling.
• A change to a higher octane fuel; a colder plug, richer fuel mixture or lower compression may be in order.
• You may also need to retard ignition timing, and check vehicle’s cooling system.
• Pre-ignition most often leads to detonation.

Above all, pre-ignition and detonation, are two separate events.

Detonation:

• The spark plugs, worst enemy! (besides fouling).
• Can break insulators or break off ground electrodes.
• Pre-ignition, most often leads to detonation.
• Plug tip temperatures, can spike to over 3000°F; during the combustion process (in a racing engine).
• Most often caused by, hot spots in the combustion chamber.
• Hot spots, will allow the air fuel mixture to pre-ignite. If the piston can’t go up (because of the force of the premature explosion). And, it can’t go down (because of the upward motion of the connecting rod). As a result,  the piston, will rattle from side to side. The resulting shock wave, causes an audible pinging sound. This is detonation.
• Most of the damage that an engine sustains when “detonating” is from excessive heat.

Misfires:

So, a spark plug can deliver, a weak spark (or no spark at all) for a variety of reasons:

• Defective coil, too much compression with incorrect plug gap.
• Dry fouled or wet fouled spark plugs, insufficient ignition timing.
• Slight misfires, can cause a loss of performance for obvious reasons.
• Severe misfires will cause poor fuel economy; poor driveability, and can lead to engine damage.

Fouling:

• Will occur when, spark plug tip temperature is; insufficient to burn off carbon, fuel, oil or other deposits.
• Will cause spark to leach to metal shell. So, no spark across plug gap, will cause a misfire.
• Wet-fouled spark plugs, must be changed or spark plugs will not fire.
• Dry-fouled spark plugs, can sometimes be cleaned. Usually, by bringing engine up to operating temperature.
• So, be sure to eliminate, the root cause of fouling; before changing fouled spark plugs.

Conclusion

So, a visual inspection of the spark plug color; will display symptoms and conditions of the engine’s performance. Furthermore, it is important to remember that spark plugs; do not create heat, they can only remove heat. The spark plug works as a heat exchanger; by pulling unwanted thermal energy, away from the combustion chamber. Finally, transferring the heat, to the engine’s cooling system.

Thank You !

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